This invention relates to cameras having shutters with electromagnetic drive sources for driving the shutter to open and close, and, more particularly, to an electromagnetically driven shutter having a mechanism for correcting the variation of shutter time due to the camera posture difference when shooting.
The electromagnetically driven shutter-equipped camera, of which the opening and closing operation of the shutter blades is performed by the electromagnetic drive source, not only can be made simpler in mechanical structure, lighter in weight and smaller in bulk and size than the mechanically driven shutter-equipped one which has prevailed in the past, but is also more advantageous in production and even can result in the reduction of production cost. Recently, therefore, many proposals have been made for such a camera.
However, the electrical power source within the camera suffers from the limitation of the electrical energy capacitance and container volume so that it is often required to limit the drive torque of this kind electromagnetic drive source to the necessary minimum.
On the other hand, with the drive torque of the electromagnetic drive source suppressed to such a low value, when the load on the electromagnetic drive source is changed, its influence will become quite serious.
And, this will lead unintentionally to differentiate the speed of movement of the shutter thereby making it impossible to assure the required accuracy of exposure control.
As an example of such situation, upon consideration of a case where, for example, the posture of the camera is changed, since the electromagnetically driven shutter mechanism is designed to have a certain load on the drive source therefor on the assumption that the camera is posed normally, when the camera is used in a different posture from the normal one, the load of the shutter mechanism on the drive source is caused to change with the required value of torque being in excess of the design value.
Now assuming that the shutter mechanism is so designed that with the camera in the normal horizontal position, the shutter blades move, for example, downwardly when the shutter is opened, then with the camera turned upside down, the opening of the shutter is effected by the upward movement of the blades with the resulting load being automatically changed. Further, since the torque of the electromagnetic drive source is suppressed to as small a value as possible, this influence works out just as it is. Thus, the shooting of the camera in the upside down position leads to a large loss in the shutter speed.